The present invention relates to a light-emitting device utilizing electroluminescence (EL).
Semiconductor lasers have been used as a light source for an optical communication system. Semiconductor lasers excel in wavelength selectivity and are capable of emitting light with a single mode. However, semiconductor lasers require many stages of crystal growth and are difficult to manufacture. Another problem with semiconductor lasers is the limitation to the types of light-emitting materials which can be used. This restricts the wavelength of light which can be emitted by semiconductor lasers.
Conventional EL light-emitting devices can emit light with a wavelength having a broad spectral width and have been applied to displays and the like. However, such EL light-emitting devices are unfit for application to optical communications and the like which require light with a narrow spectral width.
An object of the present invention is to provide a light-emitting device which can emit light with a wavelength having a remarkably narrow spectral width in comparison with conventional EL light-emitting devices, exhibits directivity, and can be applied to not only displays but also optical communications and the like.
A light-emitting device according to the present invention comprises:
a light-emitting layer being capable of emitting light by electroluminescence;
a pair of electrode layers for applying an electric field to the light-emitting layer; and
an optical waveguide for transmitting light emitted from the light-emitting layer,
wherein a grating is formed in the optical waveguide.
According to this light-emitting device, electrons and holes are introduced into the light-emitting layer respectively from the pair of electrode layers (cathode and anode). Light is emitted when the molecules return to the ground state from the excited state by recombination of the electrons and holes in the light-emitting layer. The light emitted from the light-emitting layer is provided with wavelength selectivity and directivity by the grating formed in the optical waveguide, specifically, a grating with alternating two medium layers each of which has a different refractive index arranged periodically.
It is preferable that the grating be a distributed feedback type or distributed Bragg reflection-type grating. Such a distributed feedback type or distributed Bragg reflection-type grating causes the light emitted from the light-emitting layer to resonate. As a result, light having wavelength selectivity, a narrow emission spectral width, and excellent directivity can be obtained. The pitch and depth of the grating are designed depending on the wavelength of the light to be emitted.
Light can be emitted with a single mode by providing the distributed feedback type grating with a xcex/4 phase shifted structure or a gain-coupled structure. xe2x80x9cxcexxe2x80x9d used herein indicates the wavelength of the light inside the optical waveguide.
A grating of a distributed feedback type having a xcex/4 phase shifted structure or a gain-coupled structure is a preferable configuration common to the light-emitting devices of the present invention. It is sufficient for the grating to achieve the above functions. The grating may be formed in any layers constituting the optical waveguide.
It is preferable that the light-emitting layer comprises organic light-emitting materials. Use of organic light-emitting materials expands selection of the materials and enables emission of light having various wavelengths in comparison with the case using a semiconductor or inorganic materials.
The aspects described below from (a) to (d) can be given as examples of such a light-emitting device.
(a) In a first aspect of the light-emitting device, the optical waveguide comprises a core layer mainly transmitting light, and a cladding layer having a refractive index lower than the refractive index of the core layer; and
the core layer comprises a layer which is different from the light-emitting layer.
A feature of this light-emitting device is that the core layer, which the light transmits mainly, is formed from a layer different from the light-emitting layer. The core layer is preferably made from materials having a refractive index higher than that of the light-emitting layer. This refractive index relationship ensures efficient introduction of the light emitted from the light-emitting layer into the core layer. The grating may be formed in the core layer. The grating may be formed in the boundary area between the cladding layer and a layer in contact with the cladding layer such as the core layer.
In the case where the light-emitting layer is an organic light-emitting layer comprising organic materials, the core layer may serve not only as a light transmitting layer, but also as at least one of a hole transport layer, electron transport layer, transparent electrode layer, and the like. The cladding layer is designed to have a refractive index lower than that of the core layer. The cladding layer may serve not only as a layer for light confinement, but also as an electrode layer, substrate, hole transport layer, electron transport layer, and the like.
(b) In a second aspect of the light-emitting device, the optical waveguide comprises a core layer mainly transmitting light, and a cladding layer having a refractive index lower than the refractive index of the core layer; and
the core layer comprises a layer including the light-emitting layer, and
the grating is formed in the optical waveguide.
A feature of this light-emitting device is that the light-emitting layer is included in the core layer which is the main light transmitting layer. The grating may be formed in the core layer. The grating may also be formed in a boundary area between the cladding layer and a layer in contact with the cladding layer, such as the core layer. In this light-emitting device, the light-emitting layer may be formed continuously or discontinuously one after another.
In the case where the light-emitting layer is an organic light-emitting layer formed by organic light-emitting materials, the core layer may further comprise at least one of a hole transport layer, an electron transport layer, a transparent electrode layer, and the like. The cladding layer may be designed to have a refractive index lower than that of the core layer. The cladding layer may serve not only as a layer for light confinement, but also as an electrode layer, a substrate, an hole transport layer, an electron transport layer, and the like.
In the light-emitting device according to this aspect, the grating may be formed by the light-emitting layer and a layer in contact with the light-emitting layer. According to the device having such a configuration, light emitted from the light-emitting layer resonates directly by the grating in the region including the light-emitting layer. As a result, light is emitted with a selected wavelength and excellent directivity.
(c) The light-emitting device as a third aspect of the present invention comprises an optical fiber section formed in one body,
wherein the optical fiber section comprises a core layer and a cladding layer, and
wherein the optical waveguide is formed continuously with at least one of the core layer or the cladding layer of the optical fiber section.
In this light-emitting device, because at least either the core layer or the cladding layer of the optical fiber section is formed in one body waveguide, light having excellent wavelength selectivity and directivity can be emitted from the light-emitting layer in the optical waveguide and supplied to the transmission system with high efficiency.
In this light-emitting device, the light-emitting layer may be included in the optical waveguide. The optical waveguide may be either continuous with the core layer of the optical fiber section or formed separately while being optically connected. Furthermore, it is preferable that the optical waveguide comprises a core-layer-continuing portion which continues from the core layer of the optical fiber section. In the case where the optical waveguide comprises such a part, light output from the optical waveguide is transmitted to the optical fiber with high efficiency. Moreover, this highly efficient optical combination can be obtained without requiring a delicate optical adjustment.
(d) In a fourth aspect of the light-emitting device, the grating has a defect and a one-dimensional periodic refractive index distribution which constitutes a photonic band gap; and
the defect is designed so that the energy level caused by the vacancy is within a specific emission spectrum.
According to this light-emitting device, electrons and holes are introduced into the light-emitting layer respectively from a pair of electrode layers (cathode and anode). Light is emitted when the molecules return to the ground state from the excited state by recombination of the electrons and holes in the light-emitting layer. At this time, light with a wavelength equivalent to the photonic band gap of the grating cannot be transmitted through the grating. Only the light with a wavelength equivalent to the energy level caused by the vacancy can be transmitted through the grating. Therefore, light with a remarkably narrow emission spectral width can be obtained with high efficiency by prescribing the width of the energy level caused by the vacancy.
A special feature of this aspect is in the structure of the grating. Specifically, the grating has the defect and the one-dimensional periodic refractive index distribution which constitutes the photonic band gap.
In this aspect, in order to confine the light and guide it in a certain direction, the grating is preferably formed in the optical waveguide comprising areas having either a high refractive index or low refractive index. For example, substrates, materials in contact with the grating or the air layer can function as the cladding layer.
In the light-emitting device according to this aspect, the light-emitting layer and the defect of the grating may have the following configuration.
(1) The light-emitting layer formed in the defect also functions as the defect.
(2) The light-emitting layer also functions as at least part of the defect and the grating.
(3) The light-emitting layer is formed in a region different from the defect.
In this aspect, the light-emitting layer is preferably comprises an organic light-emitting layer formed by organic material. Use of such an organic light-emitting layer is preferred to the photonic band gap using semiconductors due to the following reasons. A grating comprising an organic light-emitting layer which constitutes a photonic band gap is not affected by the irregular state of the boundary area of the light-emitting layer and impurities in comparison with the case of using semiconductors, whereby excellent characteristics from the photonic band gap can be obtained. Furthermore, in the case of forming a medium layer from an organic light-emitting layer, the manufacture becomes easy and a good periodic structure with a refractive index can be easily obtained, whereby superior characteristics from the photonic band gap can be obtained.
Some of the materials which can be used for forming each section of the light-emitting device according to the present invention will be illustrated below. These materials are only some of the conventional materials. Materials other than these materials can also be used.
(Light-emitting Layer)
Materials for the light-emitting layer are selected from conventional compounds to obtain light with a prescribed wavelength. Any of organic and inorganic compounds may be used as the materials for the light-emitting layer. It is preferable to use organic compounds from the viewpoint of a wide variety of compounds and film-formability.
As examples of such organic compounds, aromatic diamine derivatives (TPD), oxydiazole derivatives (PBD), oxydiazole dimers (OXD-8), distyrarylene derivatives (DSA), beryllium-benzoquinolinol complex (Bebq), triphenylamine derivatives (MTDATA), rubrene, quinacridone, triazole derivatives, polyphenylene, polyalkylfluorene, polyalkylthiophene, azomethine zinc complex, polyphyrin zinc complex, benzooxazole zinc complex, and phenanthroline europium complex which are disclosed in Japanese Patent Application Laid-open No. 10-153967 can be given.
Specific examples of materials for the organic light-emitting layer include compounds disclosed in Japanese Patent Application Laid-open No. 63-70257, No. 63-175860, No. 2-135361, No. 2-135359, No. 2-152184, No. 8-248276 and No. 10-153967. These compounds can be used either individually or in combinations of two or more.
As examples of inorganic compounds, ZnS:Mn (red region), ZnS:TbOF (green region), SrS:Cu, SrS:Ag, SrS:Ce (blue region), and the like can be given.
(Optical Waveguide)
The optical waveguide comprises a core layer and a cladding layer having a refractive index lower than that of the core layer. Conventional inorganic and organic materials can be used for the core layer and cladding layer.
Typical examples of inorganic materials include TiO2, TiO2xe2x80x94SiO2 mixture, ZnO, Nb2O5, Si3N4, Ta2O5, HfO2, and ZrO2 disclosed in Japanese Patent Application Laid-open No. 5-273427.
Typical examples of organic materials include various conventional resins such as thermoplastic resins, thermosetting resins, and photocurable resins. These resins are appropriately selected depending on a method of forming layers and the like. For example, in the case of using a resin which can be cured by energy of at least either heat or light, commonly used exposure devices, baking ovens, hot plates, and the like can be utilized.
As examples of such materials, a UV-curable resin disclosed in Japanese Patent Application No. 10-279439 applied by the applicant of the present invention can be given. Acrylic resins are suitable as such a UV-curable resins. UV-curable acrylic resins having excellent transparency and capable of being cured in a short period of time can be produced using various commercially-available resins and photosensitizers.
As specific examples of basic components of such UV-curable acrylic resins, prepolymers, oligomers, and monomers can be given.
Examples of prepolymers or oligomers include acrylates such as epoxy acrylates, urethane acrylates, polyester acrylates, polyether acrylates, and spiroacetal-type acrylates, methacrylates such as epoxy methacrylates, urethane methacrylates, polyester methacrylates, and polyether methacrylates, and the like.
Examples of monomers include monofunctional monomers such as 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, N-vinyl-2-pyrrolidone, carbitol acrylate, tetrahydrofurfuryl acrylate, isobornyl acrylate, dicyclopentenyl acrylate, and 1,3-butanediol acrylate, bifunctional monomers such as 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, neopentyl glycol diacrylate, neopentyl glycol dimethacrylate, ethylene glycol diacrylate, polyethylene glycol diacrylate, and pentaerythritol diacrylate, and polyfunctional monomers such as trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, pentaerythritol triacrylate, and dipentaerythritol hexaacrylate.
The above examples of inorganic and organic materials are illustrated only in consideration of light confinement. In the case where at least one layer among the light-emitting layer, hole transport layer, electron transport layer, and electrode layer functions as the core layer or the cladding layer, materials constituting these layers can be employed as materials for forming the core layer or the cladding layer constituting the optical waveguide.
(Hole Transport Layer)
As materials for the hole transport layer which is optionally formed, materials conventionally used as hole injection materials for photoconductive materials or a hole injection layer for organic light-emitting devices can be selectively used. As the materials for the hole transport layer, any organic or inorganic materials which have a function of either hole introduction or electron barrier characteristics are used. Materials disclosed in Japanese patent Application Laid-open No. 248276/1996 can be given as specific examples of such materials.
(Electron Transport Layer)
Materials for the electron transport layer which is optionally formed are required to transport electrons introduced from the cathode to the organic light-emitting layer and can be selected from conventional materials. Materials disclosed in Japanese Patent Application Laid-open No. 248276/1996 can be given as specific examples of such substances.
(Electrode Layer)
As the cathode, electron injectable metals, alloys, electrically conductive compounds with a small work function (for example, 4 eV or less), or mixtures thereof can be used. Materials disclosed in Japanese Patent Application Laid-open No. 248276/1996 can be given as specific examples of such electrode materials.
As the anode, metals, alloys, electrically conductive compounds with a large work function (for example, 4 eV or more), or mixtures thereof can be used. In the case of using optically transparent materials as the anode, transparent conductive materials such as CuI, ITO, SnO2, and ZnO can be used. In the case where transparency is not necessary, metals such as gold can be used.
In the present invention, the grating can be formed by conventional methods without specific limitations.
Typical examples of such methods are given below.
1) Lithographic Method
In this method, a positive or negative resist is irradiated with ultraviolet rays, X-rays, or the like. The resist layer is patterned by development to form a grating. As a patterning technology using a polymethyl methacrylate resist or a novolak resin resist, for example, technologies disclosed in Japanese Patent Applications Laid-open No. 6-224115 and No. 7-20637 can be given.
As a technology of patterning polyimide by photolithography, for example, technologies disclosed in Japanese Patent Applications Laid-open No. 7-181689 and No. 1-221741 can be given. Furthermore, Japanese Patent Application Laid-open No. 10-59743 discloses a technology of forming a grating of polymethyl methacrylate or titanium oxide on a glass substrate utilizing laser ablation.
2) Formation of Refractive Index Distribution by Irradiation
In this method, the optical waveguide section of the optical waveguide is irradiated with light having a wavelength which produces changes in the refractive index to periodically form areas having different refractive indices on the optical waveguide section, thereby forming a grating. As such a method, it is preferable to form a grating by forming a layer of polymers or polymer precursors and polymerizing part of the polymer layer by irradiation or the like to periodically form areas having a different refractive index. Such a technology is disclosed in Japanese Patent Applications Laid-open No. 9-311238, No. 9-178901, No. 8-15506, No. 5-297202, No. 5-32523, No. 5-39480, No. 9-211728, No. 10-26702, No. 10-8300, and No. 2-51101.
3) Stamping Method
A grating is formed by, for example, hot stamping using a thermoplastic resin (Japanese Patent Application Laid-open No. 6-201907), stamping using a UV curable resin (Japanese Patent Application Laid-open No. 10-279439), or stamping using an electron-beam curable resin (Japanese Patent Application Laid-open No. 7-235075).
4) Etching Method
A thin film is selectively patterned using lithography and etching technologies to form a grating.
Methods of forming a grating is described above. A grating consists of two areas each of which has a different refractive index. Such a grating can be formed by forming such two areas from two materials having different refractive indices, by partially modifying one material to form two areas having different refractive indices, and the like.
Each layer of the light-emitting device can be formed by a conventional method. For example, the light-emitting layer is formed by a suitable film-forming method depending on the materials. A vapor deposition method, spin coating method, LB method, ink jet method, and the like can be given as specific examples.